The present invention relates to a monolithic input stage for an optical receiver comprising a PNIN photodiode and a field effect transistor connected thereto.
In the past, avalanche photodiodes were usually used in the input stages of broadband optical receivers for fiberoptic transmission systems because of their relatively high sensitivity. The regulation of the avalanche amplification of such photodiodes can be realized only with very complicated circuitry. Moreover, it is necessary to capacitively block the avalanche photodiode against its subsequent amplifier circuit due to the high supply voltage required for the avalanche diode. However, this measure brings about undesirable stray reactances which result in a reduction of the bandwidth of the receiver.
Two publications, D. R. Smith et al, "Receivers for Optical Communications; A Comparison of Avalanche Photodiodes With PIN-FET Hybrids," Optical & Quantum Electronics, Vol. 10, 1978, pages 293-300, and R. C. Hooper et al, "PIN-FET Hybrid Optical Receivers For Longer Wavelength Optical Communication Systems," 6th European Conference on Optical Communications, York, 1980, present proof that it is possible to realize at least as high a receiver sensitivity with a PIN photodiode and a connected field effect transistor (FET) in the input of an optical receiver as with an avalanche photodiode, with the bandwidth being the same. Additionally, PIN photodiodes are voltage compatible with the connected active semiconductor components (bipolar and field effect transistors etc.), i.e., no blocking measures are required which could have a negative effect on the bandwidth of the receivers. Moreover, PIN photodiodes, in contrast to avalanche photodiodes, require no expensive regulating circuit to stabilize the diode blocking voltage.
In order to avoid the use of an avalanche photodiode in an optical receiver but nevertheless realize a high input sensitivity, a number of requirements must be met by the PIN photodiode and the field effect transistor in the input stage. Initially, a very low capacity PIN photodiode with high quantum yield and low dark current must be selected. Moreover, the field effect transistor should have the lowest possible gate leakage current, a large transconductance and a low gate-source capacitance. Finally, care must be taken that reactances which are formed due to the electrical connection between the PIN photodiode and the field effect transistor are kept as low as possible to be able to further raise the limit frequency of the receiver and thus enable its use for the highest bid rates.